The present application relates generally to gear systems for transmitting torque and, more particularly, to worm drives.
Worm drives have been used for many years. FIG. 1 shows an example of a prior art worm drive 100. The worm drive 100 includes a worm screw 102 and a worm wheel 104. The worm screw 102 includes a threaded portion 106 that meshes with teeth 108 extending around the worm wheel 104.
One advantage of worm drives is that they can produce a very low gear ratio and provide a large torque multiplication. Another advantage of worm drives is that they can act as a brake, as the output shaft cannot drive the input shaft.
A significant disadvantage of worm drives is that they generally require very precise positioning of the worm wheel relative to the worm screw to work. Any variation side to side or up and down will affect tooth engagement and can lead to premature failure.
In addition, the frictional force between the worm screw and the worm wheel teeth imparts a side load on the worm screw. This load is transmitted to the worm shaft and to its support bearings.
There are many applications where a worm drive system could be advantageously used, but the requirement of high precision makes it impractical. Also, the side loads generated require the drive system to have larger bearings than would otherwise be needed. For example, a simple lifting mechanism could benefit by a worm drive system because of the low gear ratio and the self-holding ability of a worm drive. Often, this type of system is driven by a small motor in an imprecise housing. The housing is not precise enough to allow this drive system to work effectively, and the small motor may not be designed for excessive side loads. This results in a gear system that is vulnerable to failure and a motor that is vulnerable to bearing failure.